The spine is made up of a superposition of vertebrae, that are normally aligned along a vertebral axis, going from the lumbar vertebrae to the cervical vertebrae, with each vertebra presenting a posterior wall from which there projects a spinous process and two side edges having walls from which there project the ribs and/or transverse processes. When an individual's spine presents abnormal curvature, the vertebrae are inclined relative to one another and relative to said vertebral axis. The lateral edges of the vertebrae situated on one side are thus closer to one another and form a concave curve, while the lateral edges on the other side appear spaced apart from one another and form a convex curve.
In order to straighten the spinal column, the lateral edges of the vertebrae on the concave side are spaced apart from one another and are taken relative to one another to a distance that is substantially equivalent to the distance between the lateral edges on the other side. Thereafter, in order to keep the vertebrae in that position relative to one another, known devices are used that have screws for insertion into the vertebrae or hooks for inserting along the inside wall of the spinal canal, associated with rods for interconnecting the screws or the hooks.
The hooks are generally inserted in pairs in each vertebra and on either side close to the pedicles, the heads of the hooks projecting from the posterior wall of a vertebra, one on either side of the spinous process. The heads may be tulip-shaped, for example, and they are suitable for receiving a rod which is secured by means of a nut screwed onto the head and bearing against the rod. Rows constituted by the heads of the hooks situated on either side of the spinous processes are interconnected and held in fixed position by two rods that are parallel to each other and to the axis of the spine.
Nevertheless, using such hooks may be difficult, since the operator is attempting to avoid harming the spinal cord that extends in the center of the spinal canal, since that could lead to paralysis for the patient.
The use of screws makes it possible to reduce the risks of such surgery. They likewise have tulip-shaped heads and they are inserted in pairs in the posterior walls of vertebrae in the pedicles on either side of the spinous processes. Thus, the screws constitute fastening points in the vertebrae for holding them relative to one another. Nevertheless, the screws are necessarily inserted into the pedicles of the vertebrae, and under certain circumstances, the pedicles may be small in size or they may be damaged.
A problem which arises is how to obtain such fastening points when it is not possible to introduce screws into the vertebrae in the curved portion, and when using hooks would be too dangerous. Flexible band systems for fastening to a vertebra can be used to solve this problem. For instance, the system shown as an example in accompanying FIG. 1 is one solution.
It comprises a connection piece 12 constituted by two jaws 20 and 22 that are coupled together at one end about an axis 24. The two jaws have recesses 25 enabling a rod 18 to be put into place and allowing a flexible member such as a band or cord to pass through, the flexible member 14 forming a loop 28 on one side of the connection piece 12 and two ends 30 and 32, which may be free ends of the flexible member, on the other side of said piece. The connection system also has a locking member 16 constituted by a screw that can be engaged in the ends of the jaws 20 and 22 remote from their hinged ends. The portions of the flexible member 14 that are engaged in the recesses are secured to the connection piece by being pinched between the walls of the recesses in the connection piece and the rod 18 when the locking member 16 is fully engaged.
It can be understood that in order to ensure that said assembly is properly fastened on a transverse process, on a rib, or on a portion of the posterior portion of a vertebra, it is necessary to exert tension on the ends 30 and 32 of the flexible member 14.
In the case of an intervertebral implant which is inserted between the spinal processes of two adjacent vertebrae, it is also necessary to secure the body of the implant to the spinal processes.
The securing of the implant body to the spinal processes is generally obtained by means of a flexible member such as flexible member 14, the two extremities of which are secured to the implant body.
Accompanying FIG. 2 illustrates such an intervertebral implant. The implant 50 comprises a body 52 provided with two opposite openings 54 to 56 to receive the spinal processes of the vertebrae. The implant further comprises a flexible member 58 forming two loops 58a and 58b adapted to pass around the spinal processes of the two vertebrae. The flexible member is secured to the implant body 52 by means of a securing device which includes a movable member 62 which can be moved with respect to the body itself by means of a control screw 64. Two portions of the flexible member are pinched between the body 52 and the movable member 62. The free extremities 58c and 58d of the flexible member extend out of the implant body 62.
Before securing the flexible member 58 to the implant, it is necessary to apply suitable tension to the extremities of the flexible member to obtain proper securing of the implant with the vertebrae.
In some cases, one extremity of the flexible member is directly secured to the body of the intervertebral implant, and tension is to be applied only to one free end of the flexible member.
In the present description, the wording “vertebral device” means any mechanical device used in connection with the treatment of the spine which is secured to a vertebra or to a rib by means of a flexible member including, for example, fastening devices and intervertebral implants.
The PCT application WO 2007/034112 describes a surgical tool for tensioning a flexible member which is used to secure a vertebral device, namely a fastening device, to a bone element and especially a vertebra or a rib.
This surgical tool is well adapted to different surgical operations but in some particular cases it is not suitable because it requires significant free space and the portion of the flexible member which cooperates with the tool is in the shape of a loop.